Increasing rear brake bias without a bias controller?

[SKYHI]

Based on the difference in pad wear rates I'm currently seeing, and some random temperature measurements post session at the track on Sunday, it's become pretty evident that the front brakes on my car are doing about 60-70% of the braking on my car. Normal solution would be to fit a bias proportioning valve, but unfortunately the class rules don't allow it, and I also have to stick with the standard calipers, so I'm looking for other ways to adjust the balance and get the rears to work a bit harder.

Most of you probably know the car, but it's an SE making about 220 rwhp, running on AD08R tyres, and doing short Time Attack session of 5 hot laps. The current pads are QFM A1RM, and they seem to be working well with the DBA T3 rotors. Holding together in the heat being put through them, not too aggressive on the rotors, still work well on the odd occasion the car is street dríven, and reasonably priced.

Only idea I have at this point is to run different compounds front and rear, but might need some advice in this regard, and hear from people who've had experience with this.

Any other ideas?

Waiting for Mark to jump in with his Winmax recommedations

[greenMachine]

I am away from my car atm, but does the NB8B/SE have the proportioning valve at the master cylinder? If so, playing with the spring in that will effect some change, only by trying will you know if it is enough. If you don't have one: do the regs prohibit only adjustable proportioning valves - maybe you could add one of these, and on the basis they are not "adjustable" still be legal …


Softer pads in the rear would be the quick solution, better would be use higher (I think) coefficient of friction pads in the rear, lower in front. Pad bite characteristics would also be a factor, at least on initial application. Have a good yarn to the manufacturers, they should be able to tailor a package for you.

[Magpie]

You can have a bias adjuster just not in the cabin. You could put it in the engine bay or on the rear circuit.

[rossint]

Doesn’t the SE have EBD and what looks like the prop valve is just a distribution block?
If this is the case in theory you’d be fighting the EBD system trying to introduce more rear bias, has anyone tried to make an adjustable valve work on an SE/EBD?

[track_addict]

viewtopic.php?t=75865

Some further info regarding installation of an adjustable valve into an ABS equipped car.


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[SKYHI]

You can have a bias adjuster just not in the cabin. You could put it in the engine bay or on the rear circuit.

You are right I just re-read the rules and it say "must not have in cabin adjustable brake bias or pedal box", so there is scope to install one in the engine bay etc.

viewtopic.php?t=75865

Some further info regarding installation of an adjustable valve into an ABS equipped car.

Thanks. Will have a read through that and see what my options are.

[The American]

is there a rule of thumb for when the potential compromise in handling is worth the additional rear bias? Or, how much bias adjustment is possible before handling compromises emerge?

[rossint]

My rule of thumb would be don’t let the rear of the car overtake the front.
Too much rear bias and the tail gets really loose and you can’t apply as much brake force without spinning. You generally tune for the biggest stop on the track, if you have in cab adjustment you can fine tune for different braking zones.
Here what too much rear looks like, setting up my Wilwood valve after a brake upgrade. I thought I had it pretty close going out it’s a bumpy brake zone, 190km down to about 45km, at this track I tune rear so I can brake hard and just feel the rear end squirming a tiny bit on the bumps.
https://youtu.be/qbqHABf-ci4

[RS2000]

viewtopic.php?t=75865

Some further info regarding installation of an adjustable valve into an ABS equipped car.

Adding a Wilwood valve to an ABS/EBD MX5 in the position shown in the video, will only allow pressure reduction, not increase.

The oem block on ABS/EBD cars is just a pipe joiner, not a proportional valve. Therefore the lines from the m/c to the abs unit are already seeing full m/c pressure. Replace the block with a W/W valve in the rear line, & you then have the ability to reduce the pressure, but not increase it. I personally wouldn't like to reduce the pressure from the m/c to the abs block, & can't see any reason for doing so.

One thing some people don't realise is that the Wilwood proportioning valve is NOT a proportioning valve. It is a simple pressure reducing valve, & is only connected to the rear brake line. Even if it was a prop valve, it does not receive signal pressure from the front brake line, & so can't proportion the rear pressure relative to the front pressure.

On non abs cars, removing the oem prop valve & installing the w/w valve, allows you to increase rear pressure (up to m/c pressure) above what the oem prop valve allowed. Up to the point set by the w/w knob, the rears are seeing m/c pressure. Further heavier braking wont increase the pressure out of the w/w valve to the rears.

[track_addict]

So, an ABS/EBD car with a w/w valve fitted and in the fully open position, should be no different to a non ABS car with a w/w valve in the same position. The rear brakes are seeing full m/c pressure at all times. It’s fair to say that full m/c pressure to the rear brakes is not good in a track/racing environment as you’re almost certain to lock the rear brakes. So you would then reduce the rear pressure accordingly to reduce the effectiveness of the rear brakes and avoid the lock up. I’m not sure it makes a difference between an EBD car and a non EBD car?

Now, I’m not sure how reducing the pressure pre ABS module will effect things if abs were to engage? Is it fair to say that it would just have a reduced effectiveness in the same way that normal braking is reduced?


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[RS2000]

So, an ABS/EBD car with a w/w valve fitted and in the fully open position, should be no different to a non ABS car with a w/w valve in the same position. The rear brakes are seeing full m/c pressure at all times. It’s fair to say that full m/c pressure to the rear brakes is not good in a track/racing environment as you’re almost certain to lock the rear brakes.

The abs/ebd block will see full m/c pressure with the w/w valve screwed fully in. I understand that the ebd will determine what pressure the brakes see.
On my non abs NB8A track car, I have the w/w valve 1 turn off fully in & don't lock the rears. I know some who have the w/w fully in, getting full m/c pressure to rears on track cars, but still don't lock rears. Every change you make to the car (c of g, suspension, ride height, aerodynamics, pads, rotors, calipers, weight reduction/addition, tyres, etc) affects front/rear brake bias, so every case would be different.

Personally, I wouldn't be reducing the pressure to the abs on any road or track car, & I'd leave the factory prop valve installed on non abs road cars.
But I'm getting cautious in my old age!

[plohl]

Throw some super aggressive rear pads in it. That willl help the rear do more.
Have tested it on my car, and you definitely have to adjust the prop valve.

[rossint]

So you would then reduce the rear pressure accordingly to reduce the effectiveness of the rear brakes and avoid the lock up. I’m not sure it makes a difference between an EBD car and a non EBD car?

On a non EBD car the Wilwood valve directly affects the pressure the rear brakes see by allowing you to shift the knee point. As has been said, on an EBD car you can only reduce what the front of the ABS sees, the EBD sits between that point and the rear brakes.
The only way I can see this working is putting the ABS on a switch and turning it off when you want to use the valve and having the valve fully open when running ABS.
Or you drop in an ABS system from a non EBD car.

[track_addict]

I’m not sure I follow you ross? How does the EBD only see what’s happening to the front brakes?

If you look at the graph from FM with the rear brake pressure on an EBD car, it’s a straight line, no knee point. I don’t see how it WOULDN’T work, adding a knee point via an adjustable valve.

Second to ALL of this, does anyone know how the EBD actually works? Does it have set wheel speed parameters which govern its operation? I.e. set for crappy road tyres so will bleed pressure off the back a bit too early. Or can in function like an effective automatic prop valve on a race track? If the later, there’s no need to add in a wilwood valve.


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[rossint]

By front of ABS I mean infront of the ABS system, not the front brakes.
I don’t know exactly how the EBD software works (and could be missing something here) but I assume it detects rear lock up via the ABS sensors on the wheels and adjusts the rear knee point dynamically based on what the wheels are doing via the ABS system. If this is the case, by adding a valve between the MC and ABS block you’re changing the pressure that goes into the ABS block but the EBD still controls what comes out of the ABS block and goes to the rear calipers based on what the wheels are doing not just how hard you’re pushing the pedal. Being a system designed for a street car rear bias will still be on the conservative side, I seriously doubt reducing pressure into the ABS block is going to trick the EBD into giving you a more aggressive rear bias.
For the ww valve to work you need to first remove the factory prop valve, the ww valve replaces it with an adjustable knee point. In an EBD car the ABS/EBD does the job of the prop valve, without disabling this system the ww valve can’t do it’s job, it would be the same as running both the ww valve and the factory prop valve on a non EBD car.